Re: [PATCH] sysctl: fix proc_dobool() usability

[Ondrej Mosnacek <omosnace@redhat.com>]

On Fri, Feb 10, 2023 at 3:58 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> Currently proc_dobool expects a (bool *) in table->data, but sizeof(int)
> in table->maxsize, because it uses do_proc_dointvec() directly.
>
> This is unsafe for at least two reasons:
> 1. A sysctl table definition may use { .data = &variable, .maxsize =
>    sizeof(variable) }, not realizing that this makes the sysctl unusable
>    (see the Fixes: tag) and that they need to use the completely
>    counterintuitive sizeof(int) instead.
> 2. proc_dobool() will currently try to parse an array of values if given
>    .maxsize >= 2*sizeof(int), but will try to write values of type bool
>    by offsets of sizeof(int), so it will not work correctly with neither
>    an (int *) nor a (bool *). There is no .maxsize validation to prevent
>    this.
>
> Fix this by:
> 1. Constraining proc_dobool() to allow only one value and .maxsize ==
>    sizeof(bool).
> 2. Wrapping the original struct ctl_table in a temporary one with .data
>    pointing to a local int variable and .maxsize set to sizeof(int) and
>    passing this one to proc_dointvec(), converting the value to/from
>    bool as needed (using proc_dou8vec_minmax() as an example).
> 3. Extending sysctl_check_table() to enforce proc_dobool() expectations.
> 4. Fixing the proc_dobool() docstring (it was just copy-pasted from
>    proc_douintvec, apparently...).
> 5. Converting all existing proc_dobool() users to set .maxsize to
>    sizeof(bool) instead of sizeof(int).
>
> Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
> Fixes: a2071573d634 ("sysctl: introduce new proc handler proc_dobool")
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>  fs/lockd/svc.c        |  2 +-
>  fs/proc/proc_sysctl.c |  6 ++++++
>  kernel/sysctl.c       | 43 ++++++++++++++++++++++++-------------------
>  mm/hugetlb_vmemmap.c  |  2 +-
>  4 files changed, 32 insertions(+), 21 deletions(-)

Gentle ping... Without this patch the new "dev.tty.legacy_tiocsti"
sysctl is unusable. This is blocking me from making selinux-testsuite
work with CONFIG_LEGACY_TIOCSTI=n:
https://lore.kernel.org/selinux/CAHC9VhQwrjwdW27+ktcT_9q-N7AmuUK8GYgoYbPXGVAcjwA4nQ@mail.gmail.com/T/

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.